Before the furrow formation the non-polar general contraction resulting in cell de-adhesion and spherulation, and the following relaxation leading to re-adhesion, flattening and spreading, are both necessary prerequisitesof a successful cytokinesis in A. proteus. The bipartition begins by sudden generation of two divergent cytoplasmic streamings at the late anaphase, always before the formation of furrow. All these facts fit better with the polar relaxation model than with the equatorial contraction model of initiating the fission of amoeba. After formation of the furrow the contractile ring gradually constricts the cytoplasmic connection between daughter cells. Endopasm flow in the connection bridge is no more bipolar but irregularly reversing; it compensates hydrostatic pressure differences between daughter cells. The final break of the connection is explained by its stretching because of the disparate locomotor activities on both sides of the furrow and owing to the cytoskeleton disassembly inside the connecting bridge.
Micrurgically isolated interphasal nuclei of Amoeba proteus, which preserve F-actin cytoskeletal shells on their surface, shrink after perfusion with imidazole buffer without ATP, and expand to about 200% of their cross-sectional area upon addition of pyrophosphate. These changes in size may be reproduced several times with the same nucleus. The shrunken nuclei are insensitive to the osmotic effects of sugars and distilled water, whereas the expanded ones react only to the distilled water, showing further swelling. The shrinking-expansion cycles are partially inhibited by cytochalasins. They are attributed to the state of actomyosin complex in the perinuclear cytoskeleton, which is supposed to be in the rigor state in the imidazole buffer without ATP, and to dissociate in the presence of pyrophosphate. Inflow of external medium to the nuclei during dissociation of the myosin from the perinuclear F-actin may be due to colloidal osmosis depending on other macromolecular components of the karyoplasm.Key words: Amoeba proteus, isolated nuclei, perinuclear cytoskeleton.
Summary. Amoeba proteus cells, strains A and C t , with well pronounced motor polarity (the polytactic and orthotactic forms) develop dense coats of discrete minipodia which anchor them to the glass substratum. The scanning electron microscopy demonstrates that minipodia are surface microprotrusions about 0.5 µm thick and up to 8 µm long, covering mainly the middle-anterior area of the ventral cell surface. They are few at the frontal zone and absent at the tail region. It means that the dense felt of discrete minipodia is located in the same region which has been earlier described as the zone of most efficient adhesion of a directionally moving amoeba. The cells without stable motor polarity and those which adhere to the glass without moving, or just start locomotion, lack areas covered by discrete minipodia; instead, minipodia are grouped in rosette contacts, which have the form of papillae composed of supporting platforms with crones of minipodia projected from them. The cells detached from the substratum by simple experimental procedures: radiate heterotactic forms produced by mechanical shocks, anucleate fragments obtained by microdissection, and amoebae in course of cation-induced pinocytosis, neither have separate minipodia nor rosette contacts. In contrast, during phagocytosis amoebae strongly adhere and produce dense sheaths of discrete minipodia extending along the whole surface up to the tail, except the naked front enclosing the phagosome. It was demonstrated by staining with the fluorescein-conjugated phalloidin that both types of adhesive structures: the discrete minipodia as well as the rosette contacts are very rich in the cytoskeletal F-actin.
Micrurgically isolated interphasal nuclei of Amoeba proteus, which preserve F-actin cytoskeletal shells on their surface, shrink after perfusion with imidazole buffer without ATP, and expand to about 200% of their cross-sectional area upon addition of pyrophosphate. These changes in size may be reproduced several times with the same nucleus. The shrunken nuclei are insensitive to the osmotic effects of sugars and distilled water, whereas the expanded ones react only to the distilled water, showing further swelling. The shrinking-expansion cycles are partially inhibited by cytochalasins. They are attributed to the state of actomyosin complex in the perinuclear cytoskeleton, which is supposed to be in the rigor state in the imidazole buffer without ATP, and to dissociate in the presence of pyrophosphate. Inflow of external medium to the nuclei during dissociation of the myosin from the perinuclear F-actin may be due to colloidal osmosis depending on other macromolecular components of the karyoplasm.
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